Herpes simplex virus (HSV) is a common and significant pathogen which causes a variety of disease processes in humans, ranging from cold and genital sores to blindness. The lifecycle of HSV has two distinct phases: lytic and latent infections. A pivotal HSV protein in determining the switch between lytic and latent infections is infected cell protein 0 (ICP0). ICP0 is a 110-KDa nuclear phosphoprotein that strongly transactivates viral gene expression, degrades cellular proteins in nuclear domain (ND) 10, and inhibits the anti-viral response of cellular interferons (IFNs). IFNs are secreted cellular immunomodulatory factors that upregulate the expression of ND10-associated proteins to limit the spread and replication of viruses. Genetics studies have indicated that the ND10-associated protein, promyelocytic leukemia (PML), plays an important role in IFN-mediated inhibition of HSV replication. Thus, it is likely that the interactions between ICP0, PML, and IFNs govern the type of infection HSV will establish. The mechanisms and domains on ICP0 and PML required in virus-host responses to infection through IFNs have been largely undetermined. The long-term objective of our studies is to understand at the molecular level how virus-cell interactions affect HSV infection. The objective of this proposal is to determine how specific motifs on ICP0 and PML modulate the virus-host response. Our central hypothesis is that ICP0 impairs the anti-viral activity of PML, which, in turn, is required for efficient viral replication. To test this hypothesis, we will use a variety of genetic, biochemical, and cell biology techniques to identify motifs on ICP0 and PML that participate in regulating HSV replication. Results from our studies are expected to lead to novel anti-viral therapies that inhibit or limit the severity of HSV diseases. For this purpose, our three Specific Aims are to: 1) Determine the contribution of PML motifs in the IFN response to HSV infection, 2) Identify domains in and sites on ICP0 that serve to counteract host defenses to infection, and 3) Determine the role of ICP0-PML interactions in modulating the cellular anti-viral response. The public health relevance of this research is that HSV infections are the primary cause of infectious blindness in western industrialized countries. From these studies, we expect to identify and characterize crucial connections between HSV (ICP0) and its host (IFN and PML) that determine the type infection HSV will establish. These results may be used to develop novel anti-HSV treatments.